The yusZ and yusX DNA sequences were first reported in 1993, but merely as putative open reading frames (Chen et al, 1993, Metalloregulation in Bacillus subtilis: isolation and characterization of two genes differentially repressed by metal ions, J Bact 175(17): 5428-5437).
In a later publication it was speculated that yusX, and an open reading frame located immediately upstream of yusX, denoted yusY, could have arisen from a frameshift mutation in a single yusXY gene. However, no further investigation was carried out and the authors of the publication concluded that the function of the gene(s) in the cell remained unknown (Kanamaru et al, 2002, Overexpression of the PepF Oligopeptidase Inhibits Sporulation Initiation in Bacillus subtilis, J Bact 184(1): 43-50).